Autotrophic picoplankton (APP) abundance, primary production and
vertical distribution were studied in the oligotrophic Lake Stechlin
(northeastern Germany) in 1994 and 1995. Within the euphotic zone APP
contributed 34% of phytoplankton biomass and 35% of
primary production. Annual average APP cell number was
209×103 cells ml−1, of which 95%
were unicellular cyanobacteria, 2% were
colonial cyanobacteria and 3% were eukaryotes. Three ecologically
and/or morphologically different groups of APP were recognized: (i)
unicellular cyanobacteria belonging to the genus Cyanobium,
(ii) eukaryotic species growing in early spring under isothermal conditions
and (iii) cyanobacteria, partly colonial species, growing in the stratified
period in the euphotic zone. Three species of eukaryotic green
algae were identified: Choricystis minor, Neocystis diplococca
and Pseudodictyosphaerium jurisii, the latter two
being colonial. This is the first
record of the occurrence of colonial eukaryotes potentially of APP
size in fresh waters. In summer picocyanobacteria were highly
productive so the low net increase rates indicate that losses must be
high. The dominant, Cyanobium population started growing in
February with maximum abundance in late April, contributing significantly
to the spring peak in phytoplankton biomass. During this
growth period, the population was evenly distributed in the 60 m water
column.
By the time the maximum biomass occurred, inorganic
nutrients had decreased below analytically detectable levels. Parallel
to
the onset of stratification a part of the population was grazed,
most probably in the microbial loop and primarily in the upper 10–15
m.
The rest of the Cyanobium population accumulated in a narrow
layer in the upper hypoliminon. The APP remaining from the spring was
persistent for much of the summer in this cold, high-nutrient
(especially nitrate)/low-light environment. Short phosphorus-turnover
times suggest that APP is probably phosphate-limited. The
stability of the thermocline and the pattern of thermocline development
in May
affected the accumulation of the APP cells in the upper
hypolimnion. Thus, this process is sensitive to the physical stability
of
the water column.